We're used to thinking about the universe in terms of what we can see. When we look at the night sky, we see stars, planets, and the Moon shining against the deep black background of space. When astronomers look at the sky with powerful telescopes, some that can detect light our eyes can't see, many more stars and galaxies show up.

All those bright objects the astronomers and we see are made of matter. We know that all matter attracts all other matter through a force called gravity. Earth's gravitational force is what keeps us on the ground, since we too are made of matter. Pole vaulters, elevators, planes, and rockets have to work hard to overcome the force of gravity. The same force that holds you and me to Earth's surface also keeps Earth-orbiting spacecraft and even the Moon from flying away. The Sun's gravity holds Earth and all the other planets, comets, and asteroids in our solar system in their orbits around it.

Scientists understand exactly how objects behave when pulled on by gravity. They have mathematical formulas to calculate orbits and the movement of objects that are attracting each other. This is how they can send a spacecraft to meet up with a particular comet at a particular moment in time, or predict exactly when Mars will be closest to Earth.

Now when astronomers look carefully at a galaxy, they can measure how fast the stars within it are moving. The motions of the stars are the result of the gravitational forces from all the other matter in the galaxy. But here is the key problem: When astronomers add up all the matter in all the stars and gas and dust visible with all different kinds of telescopes, it doesn't total nearly enough to explain the motions they observe. The stars are moving around much faster than they should be! In other words, all the matter we can see is not enough to produce the gravity that is pulling things around. This problem shows up over and over again almost wherever we look in the universe. Not only do stars in galaxies move around faster than expected, but galaxies within groups of galaxies do too. In all cases, there must be something else there, something we can't see, something dark.

This mismatch between what we see and what we know must be there may seem very mysterious, but it is not hard to imagine. You know that people can't float in mid-air, so if you saw what looked like a man doing just that, you would know right away that there must be wires holding him up, even if you couldn't see them.

The name scientists have given to the missing material is "dark matter." We can see the bright matter, like stars, but we know some other matter is there because of how it pulls on the bright matter. The black background of space that we tend to ignore when we enjoy the beautiful sights of the night sky really isn't as empty as you might think. As surprising as it might seem, there is more than 50 times more dark matter than bright matter in the universe.

But what exactly is the dark matter made of? Black holes and objects just a little too small to be stars (so they can't produce light of their own) make up some of it. But all of that adds up to less than one-fifth the total amount of dark matter that must be out there. We believe most of the dark matter is composed of new particles smaller than atoms that are different from anything scientists have ever detected and studied. It seems that the very essence of most of the matter in the universe is different from what you and I and rocks and trees and Earth and the Sun are made of.

Trying to understand dark matter is one of my favorite topics in science, so I'm glad I had the chance to shed some light on it for you. You can construct a cool mobile of bright galaxies out of ordinary matter. Visit [here] The Space Place at spaceplace.nasa.gov, and click on "Make spacey things."

Thanks again for calling Dr. Marc at The Space Place.